That's not a program you're going to find by default on the Start menu. To access Certificate Manager, click the Start button, type certmgr.msc in the search field, and click the Enter key.
If this is a program you use frequently, you can add it to your Start menu. Click Start, type certmgr.msc in the search field (but don't click enter). Certmgr will appear at the top of the results pane. Simply right-click on certmgr and click Pin to Start menu. It will now appear for you in the Start menu as certmgr.
Barbara